During the operation of a fluid filter, for example as a fuel filter for diesel fuel, air bubbles are to be avoided, which could be conveyed to the internal combustion engine. At the beginning of the liquid feed into an outer chamber of a filter arrangement, a certain amount of air upstream from the liquid is transported into the outer chamber. This amount of air is likewise pressed by the subsequent pressure of the liquid through the filter insert and thus also reaches the outlet of the filter arrangement. This is very disadvantageous, in particular in use as a fuel filter, because the downstream units, such as an internal combustion engine, can be hindered during operation by air pockets in the fuel supply.
For this reason, a vent should be provided in the fluid filter. By means of this vent, and in particular by means of a bore provided thereby, which is also designated as a ventilation nozzle, however there also results a permanent volume flow loss of fuel, which is conveyed directly back into the tank along with the air bubbles. In practice, for example, holes with diameters up to 0.7 mm are typical. However, the volume flow loss through these comparatively large bores should be minimized.
EP 1 356 199 B1 therefore proposes connecting two ventilation nozzles in series, i.e. providing two bores, which are arranged in series in the flow direction of the air bubbles and the fuel flow loss. The ventilating function is realized via a tube fixed on the filter housing having a first bore, which serves to receive the filter element, which is closed with a lid with two bores. In the ventilating function, both ventilation nozzles are thereby connected in series. In this way, an increased resistance for the liquid results in comparison with use of only one equally large ventilation nozzle. This increased resistance could be achieved in the use of only one ventilation nozzle through a reduction of the diameter of the bore, wherein, however, this may be costly from a manufacturing perspective and additionally may entail the risk that the especially small bore clogs quickly, so that the ventilating function is no longer provided.